1. Field of the Invention
The present invention relates to a circuit for controlling a discharge amount of a hydraulic pump which is capable of obtaining and supplying a constant flow amount of hydraulic fluid in a hydraulic pump supplied to an actuator such as a hydraulic cylinder which is adapted to drive a work apparatus for a construction heavy equipment, irrespective of a load pressure of a hydraulic cylinder, and in particular to a circuit for controlling a discharge amount of a hydraulic pump which is capable of constantly supplying a discharge amount of a hydraulic pump to a hydraulic cylinder irrespective of a load pressure of a hydraulic cylinder even when an engine operates at a low speed.
Among the terminologies used herein, the terminology “Negative system” represents a method for decreasing a discharge amount of a variable displacement hydraulic pump in the case that pilot pressure discharged from an upper stream of a pilot signal generation unit installed in a down stream of a center bypass path is high and increasing a discharge amount of a hydraulic pump in the case that the pilot pressure is low.
In addition, the terminology “Positive system” represents a method for increasing a discharge amount of a variable displacement hydraulic pump in the case that pilot pressure applied to a directional switching valve adapted to control hydraulic fluid supplied to a hydraulic cylinder is high and decreasing a discharge amount of a hydraulic pump in the case that pilot pressure is low.
2. Description of the Background Art
FIG. 1 is a schematic view illustrating a discharge amount control circuit for a hydraulic pump according to a conventional art, and FIG. 2 is a graph illustrating a discharge amount relation of a hydraulic pump based on a spool stroke in a conventional art.
As shown in FIG. 1, a variable displacement hydraulic circuit for controlling a discharge amount of a hydraulic pump according to a conventional art includes a variable displacement hydraulic pump 20 which is connected with an engine and is driven thereby, a hydraulic cylinder 24 which is connected to the hydraulic pump 20 through a supply flow path 22 and has a load W, a center bypass type directional switching valve 26 which is installed in a flow path between the hydraulic pump 20 and the hydraulic cylinder 24 and is adapted to control a start, stop and directional switch of the hydraulic cylinder 24 during a switching operation, a discharge amount adjusting valve 32 which is installed in a flow path 30 communicating with the center bypass path 28 of the directional switching valve 26 and controls a center bypass discharge amount Q2, and a pilot signal generation unit 36 which is formed of an orifice and low pressure relief valve (not shown) and is installed in a down stream of the discharge amount adjusting valve 32 and controls a discharge amount Q1 of the hydraulic pump 20.
In the drawings, reference numeral 34 represents a valve spring for adjusting an opening degree of the discharge amount adjusting valve 32. In addition, 38 represents a main relief valve for preventing the pressure in the hydraulic circuit from exceeding a set level, and 40 represents a hydraulic tank.
The discharge amount adjusting valve 32 is opened and closed by a difference pressure ΔP between an upper stream pressure (referred to a discharge pressure of the hydraulic pump 20) of the center bypass path 28 and a down stream pressure (referred to an operation pressure of the hydraulic cylinder 24) and an elastic force of the valve spring 34. The discharge amount adjusting valve 32 is set and controlled based on an opening area of the center bypass path 28.
Namely, in the case that the opening area of the opening and closing portion of the center bypass path 28 is high (when the difference pressure ΔP is small), the discharge amount adjusting valve 32 is fully largely opened by the valve spring 34. In the case that the opening area is small (the difference pressure ΔP is high), the opening area is decreased, while resisting to an elastic force of the valve spring 34.
Therefore, the bypass discharge amount Q2 in the center bypass path 28 is set to a constant discharge amount of hydraulic fluid corresponding to the opening area of the opening and closing portion of the center bypass path 28 irrespective of the pressure of the supply flow path 22.
In addition, in the case that the pilot pressure discharged from the upper stream of the pilot signal generation unit 36 is high, the pilot signal generation unit 36 is controlled in order for the discharge amount Q1 of the variable displacement hydraulic pump 20 to be decreased, and on the contrary in the case that the pilot pressure is low, the pilot signal generation unit 36 is controlled in order for the discharge amount Q1 of the hydraulic pump 20 to be increased (Referred to Negative system).
At this time, the pilot pressure which controls the discharge amount of the variable displacement hydraulic pump 20 is set in order for the discharge amount of the hydraulic pump 20 to be minimum during a neutral state of the spool of the directional switching valve 26.
As shown in FIG. 2, the discharge amount Q3 supplied to the hydraulic cylinder 24=[Discharge amount Q1 of the variable displacement hydraulic pump 20]−[Bypass discharge amount Q2 of the center bypass path 28]. Namely, the discharge amount Q3 of hydraulic fluid supplied to the hydraulic cylinder 24 is controlled to have a constant discharge amount corresponding to the opening area of the opening and closing portion of the bypass path 28 based on the position of the spool stroke of the directional switching valve 26 and is set irrespective of the load pressure of the hydraulic cylinder 24.
In the discharge amount Q1 of hydraulic fluid from the variable displacement hydraulic pump 20, as only hydraulic fluid having the discharge amount Q3 except for the bypass discharge amount Q2 of hydraulic fluid discharged to the hydraulic tank 40 through the center bypass path 28 is supplied to the hydraulic cylinder 24, in the high speed mode of the engine, a constant discharge amount Q2 of hydraulic fluid discharged from the hydraulic pump 20 is bypassed to the hydraulic tank 40, and the hydraulic fluid having the remaining discharge amount Q3 is supplied to the hydraulic cylinder 24 by a desired amount.
However, in the conventional circuit for controlling a discharge amount of the hydraulic pump, in the low speed mode of the engine, since it is impossible to fully supply the remaining discharge amount Q3 of hydraulic fluid to the hydraulic cylinder 24 by a desired amount after a constant discharge amount Q2 of hydraulic fluid discharged from the hydraulic pump 20 is bypassed to the hydraulic tank 40, a discharge amount lacking phenomenon occurs. Therefore, a cavitation occurs. In addition, since the spool strokes of the switching valve are switched more in order to obtain a discharge amount by the lacking discharge amount, an inching control interval is decreased, and an operation ability of equipment is decreased.